1. Field of the Invention
The present invention relates to a high voltage direct current transmission which comprises a rectifier station and an inverter station each connected to an AC network and connected together by a DC line.
2. The Prior Art
More particularly, the invention relates to devices for limiting overvoltages when starting the transmission, either in the case of a new start or in the case of reconnection after temporary interruptions, for example because of the release of fault protection devices. Particularly, in the latter case it is often desirable to have a quick reconnection and start of the transmission, and it is therefore of interest to achieve a rapid start control of the transmission. Such a rapid start control, however, involves a problem since it may lead to dangerous overvoltages on the line and in the rectifier station when starting the station towards a so-called "open" line.
An open line is the same as an interrupted line, which may be due to faults in the line itself, but more often it is due to faults in the inverter station, so that the inverter station does not take up any current. Since no current flows, the current control of the rectifier station will reduce the control angle for the valves of the station to a minimum, and thus the direct voltage of the station to a maximum, and as a further consequence of no or very low direct current a so-called top rectification will occur, that is, by charging of all capacitances of the transmission the direct voltage will correspond to the amplitude value of the alternating voltage without causing the voltage smoothing which arises when a current flows. To this is to be added the voltage reflection on the line, which is an inevitable consequence of the open line. This is particularly dangerous in the case of cable transmissions, which are very sensitive to overvoltages, so that this phenomenon places heavy demands on the discharge protection of the line.